When replacing magnesium (Mg), potassium (K), and calcium, which electrolyte should be replaced first?

Electrolyte Replacement Order: Magnesium First, Then Potassium, Then Calcium

When replacing electrolytes, magnesium should be corrected first, followed by potassium, and then calcium. 1

Rationale for Replacement Order

Magnesium First

• Magnesium deficiency must be corrected before attempting to correct potassium deficiency in cases of combined deficiencies 1
• Primary disturbances in magnesium balance, particularly magnesium depletion, produce secondary potassium depletion 2
• Magnesium deficiency leads to:
  • Inability of cells to maintain normal high intracellular potassium concentration
  • Increased membrane permeability to potassium
  • Possible inhibition of Na+-K-ATPase
  • Cellular potassium loss that is excreted in urine

Potassium Second

• Hypomagnesemia occurs in approximately 42% of patients with hypokalemia 1
• Repletion of cellular potassium requires correction of magnesium deficit first 2
• The European Society of Cardiology recommends addressing magnesium deficiency first to effectively correct potassium deficit in patients with hypokalemia and hypomagnesemia 1

Calcium Last

• Calcium replacement should follow magnesium and potassium correction
• Magnesium is essential to the activity of Mg-dependent adenyl-cyclase involved in both PTH release and activity on bone 3
• In magnesium deficiency, there is both deficient PTH release and peripheral resistance to PTH with subsequent hypocalcemia 3

Clinical Approach to Electrolyte Replacement

Step 1: Correct Magnesium

• For severe hypomagnesemia:
  • Initial IV magnesium sulfate dose administered over 15-30 minutes
  • Maintenance dose of 1-2 grams every 6 hours until serum magnesium normalizes 1
  • Target serum magnesium level >0.6 mmol/L 1
• Monitor:
  • Vital signs during IV administration
  • Signs of magnesium toxicity (hypotension, flushing, respiratory depression, loss of deep tendon reflexes)
  • Recheck magnesium levels 24-48 hours after initiating supplementation 1

Step 2: Correct Potassium

• After magnesium levels begin to normalize:
  • Potassium chloride is recommended for treating hypokalemia, with dosing of 20-60 mEq/day 1
  • Target serum potassium in the 4.0-5.0 mmol/L range 1
  • For life-threatening ventricular arrhythmias, maintain serum potassium above 4.0 mmol/L 1

Step 3: Correct Calcium

• After addressing magnesium and potassium deficiencies:
  • Use organic calcium salts for supplementation to prevent precipitation 3
  • Monitor both calcium and phosphorus levels simultaneously 3
  • The adequacy of calcium intake can be adjusted until both calcium and phosphorus start being excreted with low urine concentrations (>1 mmol/L) 3

Special Considerations

For Patients on Kidney Replacement Therapy

• Electrolyte abnormalities are common in patients with kidney failure receiving kidney replacement therapy (KRT) 3
• Use dialysis solutions containing appropriate potassium, phosphate, and magnesium to prevent electrolyte disorders during KRT 3
• Intravenous supplementation of electrolytes in patients undergoing continuous KRT is not recommended 3

Common Pitfalls to Avoid

1. Attempting to correct potassium before magnesium: This often fails as cellular potassium uptake remains impaired until magnesium is repleted 2
2. Administering IV magnesium too rapidly: Should not exceed 150 mg/minute as this can lead to hypotension 1
3. Neglecting to have calcium available: When giving IV magnesium, always have calcium available to reverse potential magnesium toxicity 1
4. Overlooking renal function: Patients with renal insufficiency require reduced doses and more frequent monitoring 1
5. Failing to monitor ECG: ECG monitoring is recommended for patients receiving IV magnesium, especially those with cardiac conditions 1

By following this sequential approach to electrolyte replacement, you can optimize cellular uptake and minimize complications associated with electrolyte imbalances.